Microsoft already did this mistake when going from Windows Phone 7 to Windows Phone 8. Even if it was more because of a technical issue (moving from Windows CE to Windows NT), it was really disappointing for the Windows Phone enthusiasts.

Now it is rumoured they are going to do the same mistake for Windows 10. They did not revealed anything yet for Windows RT, except this laconic answer:

We are working on an update for Surface, which will have some of the functionality of Windows 10. More information to come.

I truly think there is something to do with Windows RT. The OS is a perfect fit for anybody that don't want to have to struggle with a full OS, managing a file system, app installation, etc... I really think that if your main usage of a computer is, browsing of your pictures, writing emails or surfing the web, this kind of device are what you need. I mean for most of the computer user, tablets like these (iPad, Surface, etc...) are what they need.

The device itself cannot be compared to anything else, if you have ever had a Surface 2 in your hand, it is really thin and lightweight, with a big enough screen to do whatever you need. And sorry, Surface Pro 3 is a great device, but it is really too big to be used fully as a tablet.

Also any body who bought an RT device will be really disappointed by Microsoft. And again they are going to lose more users on this one. Even if they don't want to make to much change (like removing the desktop, which I think could really help RT devices, but that's another subject), they already have an ARM version of Windows 10 for Phone, so they could bring it to Surface.